Artificial discs

ABSTRACT

A four-component artificial intervertebral disc may provide six degrees of movement: flexion, extension, lateral bending, axial rotation, axial deflection, and anterior/posterior translation. The disc may include a superior endplate, a superior core, an inferior core, and an inferior endplate. The superior endplate may include a concave mating surface, and the inferior endplate may include a spherical mating surface. The superior endplate may roll across the superior core to provide flexion, extension, and lateral bending. The superior endplate may twist or rotate atop the superior core to provide axial rotation, and the superior endplate may slide over the superior core to provide anterior/posterior translation. The superior core may be connected to the inferior core, and the inferior core may be connected to the inferior endplate. The inferior core may be made from a flexible material that may enable the artificial disc to expand or compress vertically.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/281,786 filed on Oct. 26, 2011, which is incorporated in itsentirety herein.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure is directed to a device for replacing a diseased ordamaged intervertebral disc. In particular, the device provides a widerrange of motion than alternative treatment options, such as, forexample, a spinal fusion. Specifically, the device may allow three ormore degrees of freedom or different types of movement.

2. Related Art

The spine is composed of several individual bones, known as vertebrae.The vertebrae vary in size, shape, and function in different regions ofthe spine. The cervical vertebrae make up the bones of the neck andprovide for much of the movement of the head. The thoracic vertebrae actas anchors for the ribs and are relatively immobile. The lumbarvertebrae, located at the base of the spine, are the largest vertebraeand allow movement and articulation of the trunk.

In between each pair of vertebrae is an intervertebral disc, whichconsists of a fibrous outer portion and a gelatinous inner portion. Thediscs allow the vertebrae to move and articulate relative to oneanother. They also act as a shock absorber when there is a blow to thespine, such as from a fall or a strike on the head. In particular, anintervertebral disc is capable of at least six different motions ordegrees of freedom: flexion, which is bending forward from the waist;extension, which is bending backward from the waist; lateral bending, orleaning sideways; axial rotation, which is turning or twisting to oneside or the other; axial deflection, which may also be known as axialcompression, vertical extension, or compression along the spine; andanterior/posterior (A/P) translation, which is when one vertebrae slidesforward or backward relative to a neighbor without changing its anglerelative to the neighboring vertebra.

The intervertebral disc can be injured as a result of aging, trauma, ordisease. The disc can become desiccated or otherwise lose or weaken instructure, a condition known as degenerative disc disorder (DDD). Aherniated disc is one that has developed a tear in the outer portion,allowing the inner portion to push out. In any case, a damaged disc nolonger permits movement as it once did. As the vertebrae move out oftheir normal, healthy position, the patient may develop chronic, and insome case debilitating, pain as nerves are compressed.

Historically, the injured disc, in a condition such as DDD or herniateddisc, can be treated with spinal fusion. Spinal fusion can also beindicated as a treatment for tumors, fractures, and conditions such asscoliosis and kyphosis. In the fusion procedure, a discectomy isperformed to remove the damaged disc and the adjacent vertebrae arephysically joined together with rods, wire, or other instrumentation. Abone graft is placed between the vertebrae, and over several months, thevertebrae grow together. A typical fusion patient does not notice anyloss in mobility because her range of motion was even more restricted bythe original condition or injury.

Nevertheless, the lack of motion between the fused vertebrae placesincreased stress on the surrounding vertebrae and intervertebral discs.This increased stress may lead to premature failure or injury to thesecomponents, requiring further treatment. In addition, a fusion proceduremay be a major operation, requiring open back surgery and a longrecovery period. For these reasons, it is typically a treatment of lastresort, reserved for severe cases or when other treatment options havefailed.

Alternatives to the open spine fusion procedure, including minimallyinvasive procedures and artificial disc replacements, are in variousstages of development and practice, but these alternatives have yet tosee widespread adoption. Minimally invasive procedures involve the useof small incisions, remote control manipulation of instruments, andobservation through an endoscope or similar device. These procedures mayresult in less trauma to the patient and improved recovery times.Minimally invasive surgery can also be used to replace an injuredintervertebral disc. Instead of fusing the vertebrae above and below adamaged or diseased disc, the disc can be replaced with an artificialdisc. Current discs may provide a greater range of motion than anequivalent fusion procedure while offering equal or better treatment ofthe condition.

Current artificial discs, however, suffer from one or more drawbacks.Some discs do not enable a full range of motion along all degrees offreedom provided by a natural intervertebral disc. Current discs may notproperly restrict motion along a degree of freedom, which may result inhyperextension and injury to the patient. A disc may not be compatiblewith minimally invasive procedures for replacing the injure disc, or adisc may only be compatible with an anterior procedure. In an anteriorprocedure, a surgeon accesses the spine through an incision in theabdomen or neck. Alternative routes for accessing the intervertebraldisc include: posterior, where the incision is made directly on thepatient's spine; transforaminal, where the incision is placed to oneside of the spine; and lateral, where the incision is on the patient'sflank.

Accordingly, there is a need for an artificial disc that enables all sixdegrees of movement, restricts movement along one or more degrees,and/or is compatible with a non-anterior surgical procedure.

SUMMARY OF THE DISCLOSURE

The disclosure meets the foregoing need and allows an artificial disc toprovide three to six degrees of freedom using, e.g., a four-part design,which results in a more natural range of movement and other advantagesapparent from the discussion provided herein.

Accordingly, one aspect of the disclosure describes an artificial discfor replacing a natural intervertebral disc. The artificial discincludes a superior endplate, a superior core, a flexible inferior core,and an inferior endplate. The superior endplate includes a bi-convexsuperior surface and a concave inferior surface. The superior coreincludes a convex superior surface configured to contact the concaveinferior surface of the superior endplate. The flexible inferior core isconnected to the superior core. The inferior endplate is connected tothe inferior core and includes a bi-convex inferior surface.

The geometry of the bi-convex superior surface may be selected so as toprovide an anatomical fit to an inferior surface of a first vertebralbody, and the geometry of the bi-convex inferior surface may be selectedso as to provide an anatomical fit to a superior surface of a secondvertebral body. The artificial disc may include first multiple serratedkeels attached to the superior surface of the superior endplate, as wellas second multiple serrated keels attached to the inferior surface ofthe inferior endplate. The first multiple serrated keel may include oneor more holes perpendicular to the longitudinal axis of the keel, andthe second multiple serrated keel may likewise include one or more holesperpendicular to the longitudinal axis of the keel. The superior surfaceof the superior endplate, including the first multiple serrated keels,may be treated with a titanium and/or hydroxyapatite plasma spraycoating. Additionally, the inferior surface of the inferior endplate,including the second multiple serrated keels, may be treated with atitanium and/or hydroxyapatite plasma spray coating. The superiorendplate, superior core, and inferior core may each be made from one ormore of the following: titanium, Cr—Co—Mo (chromium, cobalt, molybdenum)alloy, or polyetheretherketone (PEEK). The inferior core may be madefrom polycarbonate urethane.

According to another aspect of the disclosure, an artificial discincludes a superior endplate and an inferior endplate. The superiorendplate includes a bi-convex superior surface and one or more serratedkeels located on the superior surface. The superior endplate alsoincludes both an anterior socket and a posterior socket for connectingto and disconnecting from a holder tool. The inferior endplate includesa bi-convex inferior surface and one or more serrated keels located onthe inferior surface. The inferior endplate also includes both ananterior socket and a posterior socket for connecting to anddisconnecting from a holder tool.

The superior multiple serrated keels may include one or more holes thatare perpendicular to the longitudinal axis of the keels. The inferiormultiple serrated keels may similarly include one or more holes that areperpendicular to the longitudinal axis of the keel. The geometry of thebi-convex superior surface may be designed to provide an anatomical fitto an inferior surface of a vertebral body. The superior surface of thesuperior endplate may include a titanium and/or hydroxyapatite plasmaspray coating. The geometry of the bi-convex inferior surface may bedesigned to provide an anatomical fit to a superior surface of avertebral body. The inferior surface of the inferior endplate mayinclude a titanium and or hydroxyapatite plasma spray coating.

The superior endplate may include a bi-convex inferior surface, and theinferior endplate may include a biconvex superior surface. The bi-convexinferior surface and the bi-convex superior surface may contact oneanother and provide a rolling/sliding, convex-on-convex articulation.The superior endplate may be made from titanium, Cr—Co—Mo alloy,ceramic, or PEEK, and the inferior endplate may be made from titanium,Cr—Co—Mo alloy, ceramic, or PEEK.

In another embodiment, the superior endplate may include a centerregion, one or more side regions, and a lip separating the center regionand the side regions. The center region of the superior endplate mayhave a thicker cross-section than the side region and may include aconcave spherical portion located in the center region. The inferiorendplate may include a center region, a side region, and a lipseparating the center region and the side region. The center region ofthe inferior endplate may have a thinner cross-section than the sideregion and may have a mating convex sphere located in the center region.The mating sphere may be structured and arranged to contact the concavespherical portion of the superior endplate when the disc is fullyassembled. The superior endplate may be made from titanium, Cr—Co—Moalloy, ceramic, or PEEK, and the inferior endplate may be made fromtitanium, Cr—Co—Mo alloy, ceramic, or PEEK.

The artificial disc may also include a first insert and a second insert.Both inserts may be made from PEEK. The first insert may connect to asocket in the superior endplate, and the second insert may connect to asocket in the inferior endplate. The first insert may include a concavemating surface, and the second insert may include a spherical matingsurface structured and arranged to connect the mating surface of thefirst insert. The superior endplate may be made from titanium orCr—Co—Mo alloy, and the inferior endplate may be made from titanium orCr—Co—Mo alloy.

In yet another aspect of the disclosure, an artificial disc includes afirst endplate, a second endplate, a first insert, and a second insert.The first endplate includes a first rail and a second rail. The secondrail is spaced a distance apart from the first rail. The first endplatefurther includes a body connected to both the first rail and the secondrail, and the body includes a socket for receiving an insert. The secondendplate is identical to the first. The first insert connects to thesocket of the first endplate, and the second insert connects to thesocket of the second endplate.

The first insert may include a concave mating surface, and the secondinsert may include a spherical mating surface. Moreover, the sphericalmating surface may be structured and arranged to contact the concavemating surface of the first insert. The first insert and the secondinsert may be made from PEEK. The first endplate and the second endplatemay be made from titanium or Cr—Co—Mo alloy.

Additional features, advantages, and embodiments of the disclosure maybe set forth or apparent from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the disclosure and the following detaileddescription are exemplary and intended to provide further explanationwithout limiting the scope of the disclosure as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the disclosure, are incorporated in and constitute apart of this specification, illustrate embodiments of the disclosure andtogether with the detailed description serve to explain the principlesof the disclosure. No attempt is made to show structural details of thedisclosure in more detail than may be necessary for a fundamentalunderstanding of the disclosure and the various ways in which it may bepracticed. In the drawings:

FIG. 1 shows an exploded view of an artificial disc that may provide sixdegrees of movement, according to one aspect of the disclosure;

FIG. 2 shows a view of the disc of FIG. 1 in an assembled state;

FIG. 3 shows a cutaway view of the assembled disc of FIG. 2;

FIGS. 4A-4D show various views of a superior endplate of an artificialdisc, according to an additional aspect of the disclosure;

FIG. 4A provides a superior view of the superior endplate;

FIG. 4B provides a lateral view of the superior endplate;

FIG. 4C provides a lateral view of the superior endplate;

FIG. 4D provides an anterior view of the superior endplate;

FIGS. 5A-5D show various views of an inferior endplate of the artificialdisc of FIGS. 4A-D;

FIG. 5A provides an inferior view of the inferior endplate;

FIG. 5B provides a lateral view of the inferior endplate;

FIG. 5C provides a lateral view of the inferior endplate;

FIG. 5D provides an anterior view of the inferior endplate;

FIG. 6 illustrates how the superior endplate of FIGS. 4A-4D may worktogether with the inferior endplate of FIGS. 5A-5D;

FIG. 7 shows an artificial disc, according to a further aspect of thedisclosure;

FIG. 8 shows an artificial disc, according to a still further aspect ofthe disclosure;

FIG. 9 shows the superior surface of the inferior endplate of theartificial disc of FIG. 8;

FIG. 10 shows an artificial disc, according to an additional aspect ofthe disclosure; and

FIG. 11 shows an exemplary insert that may be used with the endplates ofFIG. 10.

DETAILED DESCRIPTION OF THE DISCLOSURE

The embodiments of the disclosure and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments and examples that are described and/orillustrated in the accompanying drawings and detailed in the followingdescription. It should be noted that the features illustrated in thedrawings are not necessarily drawn to scale, and features of oneembodiment may be employed with other embodiments as the skilled artisanwould recognize, even if not explicitly stated herein. Descriptions ofwell-known components and processing techniques may be omitted so as tonot unnecessarily obscure the embodiments of the disclosure. Theexamples used herein are intended merely to facilitate an understandingof ways in which the disclosure may be practiced and to further enablethose of skill in the art to practice the embodiments of the disclosure.Accordingly, the examples and embodiments herein should not be construedas limiting the scope of the disclosure, which is defined solely by theappended claims and applicable law. Moreover, it is noted that likereference numerals represent similar parts throughout the several viewsof the drawings.

According to an aspect of the disclosure, an artificial disc may becapable of providing all six of the six degrees of movement seen innatural intervertebral discs. An example of this type of artificial discis shown in an exploded view in FIG. 1. Artificial disc 100 may include4 components: superior endplate 110, superior core 120, inferior core130, and inferior endplate 140. The superior endplate 110, superior core120, and inferior endplate 140 may each be made from a rigid,bio-compatible material such as, e.g., titanium or polyetheretherketone(PEEK). Components that contact bone, including the endplates, may betreated with a titanium and/or hydroxyapatite plasma spray coating. Thiscoating may serve to encourage bony on-growth, improving the strengthand stability of the connection between the component and the underlyingbone. For example, an endplate 110, 140 may be treated with a titaniumand/or hydroxyapatite plasma spray coating to foster bony on-growth andstrengthen the connection or interface between the endplate 110, 140 andthe vertebral body to which it is attached. The inferior core 130 may bemade from a polymer, such as, e.g., polycarbonate urethane (PCU), thatmay allow the disc 100 to be compressed along its vertical axis.

Axes 101 may represent the three-dimensional orientation of theartificial disc 100. For example, the axis marked X may be approximatelyaligned with or parallel to an anterior/posterior axis of the disc 100.The Y axis may be roughly parallel to a superior/inferior axis of thedisc 100. This axis may also be somewhat indicative of the vertical axisof the spine, which may be commonly referred to simply as the axis ofthe spine. The Z axis may be approximately parallel to a lateral axis ofthe artificial disc 100.

The superior endplate 110 may have an upper or superior surface 111. Thesuperior surface 111 may be bi-convex, i.e. curved from left to rightand front to back. This curvature may give the surface 111 a shape akinto a partial dome or sphere. The curvature may be complementary to thenatural curvature of an endplate of a vertebral body and may provide foran anatomical fit between the surface 111 and the vertebral body (notshown). One or more serrated keels 112 may be located on superiorsurface 111. Each keel 112 may have a longitudinal axis that is roughlyaligned along an anterior/posterior axis of the disc 100. Once adiscectomy has been completed, removing the damaged natural disc, agroove or channel may be cut into the vertebral body (not shown) toreceive each keel 112. Each keel 112 may have one more holes (not shown)perpendicular to the longitudinal axis of the keel. These holes providean aperture for bony in-growth, which may strengthen the connection orinterface between the endplate and the vertebral body.

The superior core 120 may have a superior mating surface 121 that isdomed or curved. The surface 121 may contact the concave inferiorsurface 113 of the superior endplate 110, as described below. Thesuperior core 120 may also have an inferior surface 122 that mates withthe inferior core 130. The superior core 120 may be made from a hardmaterial suitable for bearing contact, such as, for example, titanium,cobalt-chromium-molybdenum (Co—Cr—Mo) alloy, ceramic, PEEK, or the like.

The inferior core 130 may be generally cylindrical in shape. Thesuperior surface 131 of inferior core 130 may be substantially concavewith a mating knob 132 for attaching to the superior core 120, asdescribed below with reference to FIG. 3. The side of inferior core 130may be divided into an upper side 133 and a lower side 135 by a matinggroove 134. The mating groove 134 may help to attach the inferior core130 to inferior endplate 140.

The inferior endplate 140 may have a mating socket 142 formed in itssuperior surface 141. The mating socket 142 may include a mating rim 143that is complementary to the mating groove 134 of the inferior core 130.The mating socket 142 may have an overall shape that is complementary tothe lower portion of inferior core 130. The inferior core 130 may fitinto and be retained by the mating socket 142. The inferior endplate 140may additionally have one or more serrated keels 144, similar to thatfound on the superior endplate 110. The keel 144 may have a longitudinalaxis that is roughly aligned along an anterior/posterior axis of thedisc 100. The keel 144 may also have one or more holes perpendicular tothe longitudinal axis of the keel to encourage bony in-growth, asdescribed above with respect to the superior endplate 110. The inferiorendplate 140 may have an inferior surface (not shown) that is bi-convex,as described above with respect to superior surface 111.

FIG. 2 illustrates a view of an assembled artificial disc 100. Thesuperior edge of keel 112 may be parallel or roughly parallel to theinferior edge of the keel 144. When fully assembled, the superiorendplate 110 and the inferior endplate 140 may be closer together on oneside and farther apart on the opposite side. The axis formed by thesetwo points may be aligned or roughly aligned with the anterior/posterioraxis of the disc 100, the longitudinal axis of the keel 112, and/or thelongitudinal axis of the keel 144. The side of disc 100, where theendplates 110, 140 are closest together may be the posterior end of thedisc 100, and the side where the endplates 110, 140 are farthest apartmay be the anterior end of the disc 100. Serrations or divisions of thekeels 112, 144 may be parallel or approximately parallel to thesuperior/inferior axis of the disc 100 (i.e. the axis of the spine).Serrations or divisions of the keels 112, 144 may be perpendicular tothe respective surface of the superior endplate 110 or inferior endplate140. Alternatively, the serrations or divisions may be angled withrespect to the superior/inferior axis of the disc 100, the axis of thespine, or the surface of the endplates 110, 140. The serrations ordivisions may be angled away from the posterior end or side of theartificial disc 100 and toward the anterior end or side. This angle mayreduce or prevent the disc 100 from moving toward the anterior, i.e.backing out of the intervertebral space and/or groove(s) cut intovertebral bodies (not shown).

FIG. 3 provides a cutaway view of the assembled disc 100, revealing theinterconnections between the four components of the disc 100. Superiorendplate 110 may include a socket 114, and inferior endplate 140 maysimilar include a socket 145. Sockets 114, 145 may receive a holder ortool to assist a surgeon in grasping the artificial disc 100 andinserting it into an intervertebral space during a surgical procedure.

The inferior surface of superior endplate 110 may include a concavemating surface 113. The mating surface 113 may contact the superior core120, as shown in FIG. 3. The curved or domed superior mating surface 121of the superior core 120 may have a smaller radius than the inferiormating surface 113 of the superior endplate 110. This arrangement may besimilar to a smaller ball placed within a larger hollow concave sphereor bi-concave channel. The contact area between the two spheres may bereduced to a single point of contact. Similarly, the contact areabetween the superior endplate 110 and the superior core 120 may bereduced to a very small area, including, e.g., a single point.

The small contact area may permit the artificial disc to provide severaldegrees of freedom. For example, by configuring the inferior matingsurface 113 of the superior endplate into a bi-convex channel, thesuperior endplate 110 may articulate or roll across the superior core120 on an anterior/posterior axis to provide for extension and flexionof the spine. Articulation or rolling along a left/right axis may permitlateral bending. Furthermore, the superior endplate 110 may rotate orarticulate on top of the superior core 120 to allow axial rotation. Inaddition, the superior endplate 110 may slide or shift relative to thesuperior core 120 along an anterior/posterior axis to emulateanterior/posterior translation. In this manner, the connection betweenthe superior endplate 110 and superior core 120 may enable four degreesof movement. In another embodiment of the design, the inferior matingsurface 113 of the superior endplate may be configured to additionallyallow the superior endplate 110 to slide or shift relative to thesuperior core 120 along a medial/lateral axis to emulate medial/lateraltranslation. In this manner, the connection between the superiorendplate 110 and superior core 120 may enable five degrees of movement.

The superior core 120 may include an inferior mating surface 122 with amating socket 123. As shown in FIG. 3, the socket 123 may include one ormore angled walls such that the socket 123 is narrower at its inferiorend and wider at its superior end. This arrangement may create a rim orlip around the opening of socket 123.

The mating socket 123 may be configured to receive a mating knob 132 onthe superior surface 131 of the inferior core 130. The mating knob 132may have a configuration that is complementary to the mating socket 123.For example, the knob 132 may be wider at its superior end and narrowerat its inferior end. The knob 132 and the socket 123 may be designedwith such dimensions that, once the components are joined, theconnection is permanent and the components cannot be separated. Thenormal biomechanical forces acting on an intervertebral disc may not beable to separate the superior core 120 and the inferior core 130.

As described above, the inferior core 130 may be made from a polymerthat is flexible, at least in comparison to materials such as titaniumand PEEK. Polycarbonate urethane (PCU) is one example of a polymer thatmay be suitable for constructing the inferior core 130. The use of aflexible material for the inferior core 130 may enable the artificialdisc 100 to have an additional degree of freedom, axial deflection oraxial compression.

To enable six degrees of freedom in movement, the inferior core 130 maybe compressed slightly as it is inserted into the intervertebral space.The inferior core 130 may then expand when the patient extends her spinevertically, mimicking the flexibility of a natural intervertebral disc.In addition, the flexible material of the inferior core 130 may allowthe artificial disc 100 to be compressed when the patient experience's aspinal compression or deflection. Again, this compression may replicatethe behavior of a natural intervertebral disc. The compression of theartificial disc 100 may help cushion the spine and vertebrae againstdamage and injury when the patient undergoes axial deflection orcompression.

The inferior core 130 may be roughly cylindrical in shape with a sidethat may be roughly straight. Alternatively, the side may be somewhatconvex. In either form, the side may be divided into an upper side 133and a lower side 135 by a mating groove 135. The mating groove 135 maybe complementary to a mating rim 143 in a mating socket 142 on theinferior endplate 140. As seen in FIG. 3, the lower wall 135 may beretained within the mating socket 142, while the upper wall 133 mayextend above the superior surface 141 of the inferior endplate 140. Thestructure and arrangement of the mating socket 142, mating rim 143,mating groove 134, and lower wall 135 may create a permanent connectionbetween the inferior core 130 and the inferior endplate 140. The normalbiomechanical forces acting on an intervertebral disc may not be able toseparate the inferior core 130 and the inferior endplate 140.

The inferior endplate 140 may have a superior surface 141, and themating socket 142, which may include the mating rim 143, may be formedin the superior surface. The inferior endplate 140 may also have aninferior surface 146. The inferior surface 146 may be bi-convex, i.e.curved from left to right and front to back. This curvature may give thesurface 146 a shape akin to a partial dome or sphere. The curvature maybe complementary to the natural curvature of an endplate of a vertebralbody and may provide for an anatomical fit between the surface 146 andthe vertebral body. One or more serrated keels 144 may be located on theinferior surface 146. Each keel 144 may have a longitudinal axis that isroughly aligned along an anterior/posterior axis of the disc 100. Once adiscectomy has been completed, a groove may be cut into the vertebralbody (not shown) to receive each keel 144. Each keel 144 may have onemore holes (not shown) that are perpendicular to the longitudinal axisof the keel 144. These holes may provide an aperture for bony in-growth,which may strengthen the connection or interface between the endplateand the vertebral body.

The artificial disc 100 may provide the same six degrees of movementfound in healthy, natural intervertebral discs: flexion, extension,lateral bending, axial rotation, anterior/posterior translation,medial/lateral translation and axial deflection. The first five degreesof movement may be enabled by the interface between the superiorendplate 100 and the superior core 120. This interface may utilize arolling and articulating sphere-on-sphere design, as described above.The sixth degree of movement, axial deflection, may also be known asaxial compression. This type of movement may be enabled by therelatively flexible material used to construct the inferior core 130.The inferior core 130 may be compressed slightly when the artificialdisc 100 is inserted in the intervertebral space, which may allow thedisc to expand when a patient's spine is lengthened or stretched. Inaddition, the inferior disc 130 may be further compressed when thepatient's spine experiences axial compression or deflection.

FIGS. 4A-4D show the superior endplate 210 of a two-component artificialdisc 200 according to an additional aspect of the disclosure. FIGS.5A-5D show the inferior endplate 220 of the two-component artificialdisc 200, and FIG. 6 illustrates how the two endplates 210, 220 may worktogether. FIG. 4A provides a superior view of the endplate 210. FIG. 4Bprovides a lateral view of the endplate 210. FIG. 4C provides anopposite lateral view of endplate 210. FIG. 4D provides a anterior viewof endplate 210.

The endplate 210 may include one or more serrated keels 212. The keel212 may have a longitudinal axis that is roughly aligned along anmedial/lateral axis of the disc 200, discussed below with respect toFIG. 6. Each keel 212 may have one or more holes (not shown) that areperpendicular to the longitudinal axis of the keel to encourage bonyin-growth from the attached vertebral body. The endplate 210 may includea socket 211 for connecting to a holder or tool (not shown) that may beused by a surgeon to insert the disc 200 into a patient's spine. Whilethe socket 211 is shown on the lateral end of the disc 200, the disc 200may alternatively or additionally include a socket at its anterior andposterior end (not shown).

The endplate 210 may have a superior surface 213 that is bi-convex. Anarrow A indicates the anterior/posterior curvature of surface 213, andan arrow C indicates the medial/lateral curvature of the surface 213.(The upper line of arrow C is tangential to the line and at the pointindicated by an arrow C′.) The bi-convex shape may be complementary to avertebral body to which the artificial disc 200 may be attached and mayprovide for an anatomical fit between the surface 213 and the vertebralbody (not shown). The curvature indicated by arrows A and C may vary fordifferent aspects of the disc 200, e.g. discs intended for use indifferent regions of the spine, or discs intended to replace differentnatural intervertebral discs.

In addition, the endplate 210 may include an inferior surface 214 thatis bi-convex. Thus, the inferior surface 214 may have a shape akin to aportion of a dome or sphere. An arrow B indicates the anterior/posteriorcurvature of surface 214, and an arrow D shows the medial/lateralcurvature of the surface 214.

FIG. 5A provides an inferior view of the endplate 220. FIG. 5B providesa lateral view of endplate 220. FIG. 5C provides an opposite lateralview of endplate 220. FIG. 5D provides an anterior view of endplate 220.The endplate 220 may have an inferior surface 224 with one or moreserrated keels 222. Each keel 222 may have a longitudinal axis that isroughly aligned along an medial/lateral axis of the disc 200, discussedbelow with respect to FIG. 6. The keel 222 may have one or more holes(not shown) that are perpendicular to the longitudinal axis of the keel.The holes may encourage bony in-growth, strengthening the attachment orinterface between the endplate 222 and a vertebral endplate (not shown).The endplate 220 may include a socket 221 for connecting to a holder ortool (not shown) that may be used by a surgeon to insert the disc 200into a patient's spine. While the socket 221 is shown on the lateral endof the disc 200, disc 200 may alternatively or additionally include asocket at its anterior and posterior end (not shown).

The inferior surface 224 may have a bi-convex shape, similar to thesuperior surface 213 of the superior endplate 210. Theanterior/posterior curvature of the inferior surface 224 is indicated byan arrow E in FIG. 5B, and the medial/lateral curvature is indicate byan arrow G in FIG. 5D. (The lower line of the arrow G is tangential tothe line and at the point indicated by arrow G′.) The curvature may becomplementary to the natural curvature of an endplate of a vertebralbody and may provide for an anatomical fit between the surface 224 andthe vertebral body (not shown). The curvature of inferior surface 224may or may not match that of superior surface 213. In particular, thecurvature shown at arrow A may be the same as that of arrow E, or thecurvature may be different. The curvature indicated at arrow C may bethe same as that of arrow G, or the curvature may be different. Thecurvatures of the inferior surface 224 and the superior surface 213 mayvary based upon a number of factors, as will be understood by thoseskilled in the art. Exemplary factors may include, without limitation,the region of the spine into which the artificial disc is inserted, aswell as the specific natural intervertebral disc that is being replaced.

The endplate 220 may have a superior surface 223 with a bi-convex shape,similar to the inferior surface 214 of superior endplate 210. Theanterior/posterior curvature is shown by an arrow F in FIG. 5C, and themedial/lateral curvature is shown by an arrow H in FIG. 5D. Thecurvature of superior surface 223 may or may not be the same as that ofinferior surface 214. The curvature indicated by arrow F may be the sameas that of arrow B, or it may be different. The curvature indicated byarrow H may be the same as that of arrow D, or it may be different.

FIG. 6 shows an assembled artificial disc 200, including superiorendplate 210 and the inferior endplate 220. Axes 201 may represent thethree-dimensional orientation of the artificial disc 200. For example,the axis marked X may be approximately aligned with or parallel to anmedial/lateral axis of the disc 200. The Y axis may be roughly parallelto a superior/inferior axis of the disc 200. The Z axis may beapproximately parallel to a anterior/posterior axis of the artificialdisc 200.

The inferior surface 214 of the superior endplate 210 may contact thesuperior surface 223 of the inferior endplate 220. Since both theinferior surface 214 and the superior surface 223 are bi-convex, thecontact area between the endplates 210, 220 may be minimal. Thisarrangement may allow the endplates 210, 220 to articulate against eachother in a rolling/sliding or sphere-on-sphere manner. In particular,the endplates 210, 220 may roll laterally to permit lateral bending, andthey may roll along an anterior/posterior axis to enable flexion andextension. In addition, the superior endplate 210 may rotate relative tothe inferior endplate 220, or vice versa, to allow axial rotation. Itshould be noted that sliding between surfaces 214 and 223 may alsooccur.

The artificial disc 200 may include sockets 211, 221 at both lateralends, or just at one lateral end. The artificial disc 200 may be madefrom metal, such as, e.g., stainless steel, titanium,cobalt-chromium-molybdenum (Co—Cr—Mo) alloy, or the like; ceramic; PEEK;or any other hard material suitable for bearing contact, as will beunderstood by one skilled in the art. Surfaces of the artificial disc200 that will contact bone, such as the superior surface 213 and theinferior surface 224, may be treated with a titanium and/orhydroxyapatite plasma spray coating to encourage bony on-growth. Thebony on-growth may act to strengthen the attachment or interface betweenthe artificial disc 200 and the underlying vertebrae.

FIG. 7 shows an artificial disc 300 according to a further aspect of thedisclosure. The artificial disc may include a superior endplate 310 andan inferior endplate 320. Each endplate 310, 320 may include a metalcomponent and a polymer insert. The metal components may be identical,so that the superior endplate 310 differs from the inferior endplate 320only in the polymer insert. The superior endplate 310 may include tworails 311 a, 311 b that are joined by a body 312, which may have asocket 313 for a polymer insert 314. Likewise, the inferior endplate 320may include two rails 321 a, 321 b that are joined by a body 322, whichmay have a socket 323 for a polymer insert 324.

Axes 301 may represent the three-dimensional orientation of theartificial disc 300. For example, the axis marked X may be approximatelyaligned with or parallel to an medial/lateral axis of the disc 300. TheY axis may be roughly parallel to a superior/inferior axis of the disc300. The Z axis may be approximately parallel to a medial/lateral axisof the artificial disc 300.

Following a discectomy, slots or holes may be cut into the endplates ofthe vertebral bodies (not shown) to receive the rails 311 s, 311 b, 321a, and 321 b. The rails, along with any other part that contacts bone,may be treated by a titanium and/or hydroxyapatite plasma spray coatingto encourage bony on-growth. Bony on-growth may enhance the stability ofthe attachment or interface between the endplate 310, 320 and itsassociated vertebra.

The endplates 310, 320 may be constructed from titanium, Co—Cr—Mo alloy,or any other metal or alloy, as will be understood by those skilled inthe art. Since the endplates 310, 320 may be identical in design, theload of the vertebral body may be equally shared on or over the entireouter surface of the artificial disc 300. This may reduce the risk fordamage or injury to the attached vertebral bodies and/or the artificialdisc 300 itself.

The superior insert 314 may include a mating sphere 315, and theinferior insert 324 may include a larger, spherical mating socket 325.The mating sphere 315 may have a radius that is smaller than the radiusof the socket 325. This arrangement may enable rolling articulationbetween the sphere 315 and the socket 325. By extension, the endplates310, 320 may have a rolling articulation, giving the artificial disc 300a more natural range of motion. For example, the endplates 310, 320 mayroll laterally to permit lateral bending, and they may roll along ananterior/posterior axis to enable flexion and extension. In addition,the superior endplate 310 may rotate relative to the inferior endplate320, or vice versa, to allow axial rotation. The superior insert 314 andthe inferior insert 324 may be made from PEEK or any like material.

FIG. 8 shows an artificial disc 400 that is constructed according to anadditional aspect of the disclosure. Axes 401 may represent thethree-dimensional orientation of the artificial disc 400. For example,the axis marked X may be approximately aligned with or parallel to anmedial/lateral axis of the disc 400. The Y axis may be roughly parallelto a superior/inferior axis of the disc 400. The Z axis may beapproximately parallel to an anterior/posterior axis of the artificialdisc 400.

The artificial disc 400 may include a superior endplate 410 and aninferior endplate 420. The superior endplate 410 may have a superiorsurface 411 and an inferior surface 412. The superior surface 411 mayinclude one or more serrated keels 415. Each keel 415 may have alongitudinal axis that is approximately parallel to a medial/lateralaxis of the disc 400. Each keel 415 may also have one or more holes 416that are substantially perpendicular to the longitudinal axis of thekeel. For example, the holes 416 may be roughly parallel to aanterior/posterior axis of the disc 400. The holes 416 may encouragebony in-growth, thereby strengthening the attachment or interfacebetween the endplate 410 and the underlying vertebral body or other bone(not shown). The superior endplate 410 may include a lateral socket 417a for receiving a tool or holder (not shown) for gripping the artificialdisc 400 during a surgical procedure.

FIG. 9 shows the inferior surface 412 of the endplate 410. The inferiorsurface 412 may be divided into two side regions 413 that are separatedby a center region 414. The cross-sectional thickness of the endplate410 may be thinner in the side regions 413 and thicker in the centerregion 414. The center region 414 and the side regions 413 may beseparated by a lip 419. The center region 414 may include a socket 418having a concave spherical or rounded shape.

FIG. 9 also shows a lateral socket 417 b for receiving a tool or holder(not shown) for use in surgery. Traditionally, an intervertebral disc,which is anterior of the spinal cord, is accessed from the patient'santerior. For procedures in the lumbar region of the spine, the surgeonmust cut and navigate through the muscles, tissues, and organs of thepatient's abdomen. In contrast, a posterior procedure accesses the discfrom an incision on the patient's back, and a transforaminal procedureaccesses the disc from an incision made to one side of the spine. Incertain cases, the lumbar region of the spine may also be accessed froma direct lateral approach through the patient's side. These proceduresmay be easier for a surgeon to perform. In addition, these proceduresmay cause less trauma to the patient and result in faster recovery.

The inferior endplate 420 may have a superior surface 421 and aninferior surface 422. The superior surface 421 may be divided into twoside regions 423 and a center region 424. The cross-sectional thicknessof the endplate 420 may be thicker in the side regions 423 and thinnerin the center region 424. This arrangement may be the opposite orinverse of the superior endplate 410. The superior surface 421 mayfurther include a mating sphere 428, which may mate with the socket 418of the superior endplate 410 when the artificial disc 400 is fullyassembled, as shown in FIG. 8. The inferior endplate 420 may have aninferior surface 422 with one or more serrated keels 425. The keel 425may have a longitudinal axis that is approximately parallel tomedial/lateral axis of the disc 400. The keel 425 may include one ormore holes 426 that are substantially perpendicular to the longitudinalaxis of the keel. For example, the holes 426 may be roughly parallel toa anterior/posterior axis of the disc 400. The holes 426 may encouragebony in-growth, thereby strengthening the attachment or interfacebetween the endplate 420 and the underlying vertebral body or other bone(not shown). The inferior endplate 420 may additionally include alateral socket 427 for receiving a tool or holder (not shown) forgripping the artificial disc 400 during a surgical procedure. Theinferior endplate 420 may likewise have an opposite lateral socket (notshown) for receiving a tool for gripping the disc 400.

The inferior endplate 420 may include a mating sphere 428, and thesuperior endplate 410 may include a larger, spherical mating socket 418.The mating sphere 428 may have a radius that is smaller than the radiusof socket 418. This arrangement may enable roll/slide articulationbetween sphere 428 and socket 418. By extension, the endplates 410, 420may have a roll/slide articulation, giving the artificial disc 400 amore natural range of motion. In particular, the endplates 410, 420 mayroll laterally to permit lateral bending, and they may roll along ananterior/posterior axis to enable flexion and extension. In addition,the superior endplate 410 may rotate relative to the inferior endplate420, or vice versa, to allow axial rotation.

The superior center region 414 may interact with the inferior sideregions 423 to limit axial rotation. When the superior endplate 410 isrotated relative to the inferior endplate 420, the lip 419 may bebrought into contact with the lip 429. This contact may prevent furtherrotation of endplate 410. The dimensions of the center regions 414, 424;the side regions 413, 423; and the lips 419, 429 may be selected so asto select a particular range of motion for a particular application ofthe disc 400. For example, a replacement disc for an intervertebral discat the top of the thoracic spine may require a different range of motionthan a replacement disc for the bottom for the thoracic spine.

The superior endplate 410 and the inferior endplate 420 may be made frommetal, such as, e.g., stainless steel, titanium, Co—Cr—Mo alloy or thelike; ceramic; PEEK; or any other hard material suitable for bearingcontact, as will be understood by one skilled in the art. Surfaces ofthe artificial disc 400 that will contact bone, such as the superiorsurface 411 and inferior surface 422, may be treated with a titaniumand/or hydroxyapatite plasma spray coating to encourage bony on-growth.The bony on-growth may act to strengthen the attachment or interfacebetween the artificial disc 400 and the underlying vertebrae.

FIG. 10 shows an artificial disc 500 according to an additional aspectof the disclosure. Axes 501 may represent the three-dimensionalorientation of the artificial disc 500. For example, the axis marked Xmay be approximately aligned with or parallel to an medial/lateral axisof the disc 500. The Y axis may be roughly parallel to asuperior/inferior axis of the disc 500. The Z axis may be approximatelyparallel to an anterior/posterior axis of the artificial disc 500.

The artificial disc 500 may include a superior endplate 510 and aninferior endplate 520. The superior endplate 510 may have a superiorsurface 511 and an inferior surface 512. One or more serrated keels 513may be positioned on the superior surface 511. Each keel 513 may have alongitudinal axis that is approximately parallel to an medial/lateralaxis of disc 500. The keel 513 may have one or more holes (not shown)that are substantially perpendicular to the longitudinal axis of thekeel, and these holes may serve to encourage bony in-growth from thesupporting vertebral body or other bone (not shown). Bony in-growth maystrengthen the attachment of the endplate 510 to the vertebral body orother bone. The superior surface 511 may be bi-convex, a shape that maybe complementary to a vertebral body or other bone and may provide foran anatomical fit between the surface 511 and the vertebral body (notshown).

The inferior surface 512 may be convex along an medial/lateral axis ofthe artificial disc 500, or it may be bi-convex. The inferior surface512 may include a socket 514 for an insert 516. The socket 514 may havea circular wall 528 that is divided into a superior portion 528 a and aninferior portion 528 b by a lip 515. The inferior portion 528 b, whichmay be closer to the inferior surface 512, may have a larger diameterthan the superior portion 528 a of the wall 528.

The inferior endplate 520 may include a superior surface 521 and aninferior surface 522. One or more serrated keels 523 may be located onthe inferior surface 522. The keel 523 may have a longitudinal axis thatis roughly parallel to a medial/lateral axis of the disc 500. The keel523 may have one or more holes (not shown) that are substantiallyperpendicular to the longitudinal axis of the keel 523. The holes mayfunction to encourage bony in-growth from an underlying bone, such as anendplate of a vertebral body. The inferior surface 522 may be bi-convexand may have a shape that is complementary to an attached underlyingbone, providing an anatomical fit between the surface 522 and thevertebral body (not shown).

Similarly, the superior surface 521 of the inferior endplate 520 mayhave a shape that is bi-convex, or it may be convex only along amedial/lateral axis of the disc 500. The superior surface may alsoinclude a socket 524 for receiving an insert 526. The socket 524 mayhave an internal design that is similar or identical to the socket 514of the superior endplate 510. For example, the socket 524 may have acircular side wall that is divided into superior and inferior portionsby a lip (not shown). The superior portion may have a wider diameterthan the lower portion. Additionally, the inferior endplate 520 may havea socket 527 for receiving a tool (not shown) to grip or hold theartificial disc 500 during surgery. While the socket 527 is shown onlyon the lateral end of the superior plate, one skilled in the art willappreciate that identical sockets may be located on superior endplate510, as well as at the opposite end of both endplates 510, 520.

FIG. 11 shows an example of an insert 526 for use with the superiorendplate 510 or inferior endplate 520. The insert 530 may be designed tosnap or fit into socket 514 or socket 524. The insert 530 may include anupper surface 531, a side 532, and a lower surface 533. The uppersurface 531 may be contiguous with an inferior surface 512 of a superiorendplate 510, or it may be contiguous with a superior surface 521 of aninferior endplate 510. The side 532 and the lower surface 533 may beplaced within the socket 514, 524. One or more tabs 534 may extend fromthe lower surface 533. The tab 533 may include a hook 535 for securingthe insert 503 in a socket 514, 524. The tab 534 may have a thincross-section at its base where it attaches to the surface 533. Thecross-section may become thicker at the hook and then taper to a pointor flat tip at the end of the tab 534 that is farthest from surface 533.The design of the tabs 534 may allow the insert 530 to be easilyattached to a socket 514, 524 but prevent easy removal. For example, theside 532 of the insert 530 may abut the inferior sidewall 528 b when theinsert 530 is placed into socket 514. The narrow base portion of tab 534may be placed against the lip 515, and the hook portion 535 of the tab534 may press against the superior sidewall 528 a. In this arrangement,the hooks 535 may be complementary to the shape of the lip 515 and thesidewall 528 a. The hooks 535 may grip the lip 515, thereby preventingthe insert 530 from leaving the socket.

While the insert 530 is shown with an upper surface 531 that has a domedor spherical shape, other arrangements and structures are contemplatedand may be used without departing from the spirit or scope of thedisclosure, including the attached claims and drawings. For example,the, upper surface 531 may have a concave rounded or spherical shape,such as the insert 516, shown in FIG. 10. Such a shape may be used tomate with the spherical insert 526, also shown in FIG. 10. Anotherpossible arrangement is the use of two spherical inserts for asphere-on-sphere arrangement.

The first arrangement, with a smaller sphere in a larger sphericalsocket, may enable roll/slide articulation between the sphere 526 andthe socket. By extension, the endplates 510, 520 may have a roll/slidearticulation, giving the artificial disc 500 a more natural range ofmotion. For example, the endplates 510, 520 may roll laterally to permitlateral bending, and they may roll along a medial/lateral axis to enableflexion and extension. In addition, the superior endplate 510 may rotaterelative to the inferior endplate 520, or vice versa, to allow axialrotation.

In the second arrangement, which may be sphere-on-sphere, the contactarea between the endplates 510, 520 may be minimal. This arrangement mayallow the endplates 510, 520 to articulate against each other in aroll/slide manner. For example, the endplates 510, 520 may rolllaterally to permit lateral bending, and they may roll along amedial/lateral axis to enable flexion and extension. In addition, thesuperior endplate 510 may rotate relative to the inferior endplate 520,or vice versa, to allow axial rotation.

The superior endplate 510 and the inferior endplate 520 may be made frommetal, such as, e.g., stainless steel, titanium, Co—Cr—Mo alloy, or thelike; ceramic; or any other hard material suitable for bearing contact,as will be understood by one skilled in the art. The Inserts 516, 526,530 may be made from PEEK or any other suitable material, as will beunderstood by one skilled in the art. The surfaces of the artificialdisc 500 that may contact bone, such as the superior surface 511 andinferior surface 522, may be treated with a titanium and/orhydroxyapatite plasma spray coating to encourage bony on-growth. Thebony on-growth may act to strengthen the attachment or interface betweenthe artificial disc 500 and the underlying vertebrae.

While the disclosure has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the disclosurecan be practiced with modifications in the spirit and scope of theappended claims. These examples given above are merely illustrative andare not meant to be an exhaustive list of all possible designs,embodiments, applications or modifications of the disclosure.

What is claimed is:
 1. An artificial disc comprising: a superior endplate comprising a bi-convex superior surface and a concave inferior surface; a superior core comprising a convex superior surface configured to contact the concave inferior surface of the superior endplate; a flexible inferior core having an upper wall and a lower wall, the flexible inferior core configured to connect to the superior core, the upper wall and the lower wall divided by a mating groove, wherein the flexible inferior core comprises a mating knob surrounded by a groove and a raised ridge on the upper wall extending around an outer periphery of the inferior core for mating with the superior core, the groove being positioned between the mating knob and the raised ridge, wherein the flexible inferior core further comprises a substantially concave superior surface within which the mating knob resides; and an inferior endplate configured to connect to the inferior core, the inferior endplate comprising a bi-convex inferior surface.
 2. The artificial disc of claim 1, wherein the geometry of the bi-convex superior surface is selected to provide an anatomical fit to an inferior surface of a first vertebral body, and the geometry of the bi-convex inferior surface is selected to provide an anatomical fit to a superior surface of a second vertebral body.
 3. The artificial disc of claim 1, further comprising: a first serrated keel located on the superior surface of the superior endplate; and a second serrated keel located on the inferior surface of the inferior endplate.
 4. The artificial disc of claim 3, wherein the first serrated keel comprises at least one hole substantially perpendicular to a longitudinal axis of the first keel, and the second serrated keel comprises at least one hole substantially perpendicular to a longitudinal axis of the second keel.
 5. The artificial disc of claim 3, wherein the superior surface of the superior endplate, including the first serrated keel, is treated with a titanium and/or hydroxyapatite plasma spray coating, and the inferior surface of the inferior endplate, including the second serrated keel, is treated with a titanium and/or hydroxyapatite plasma spray coating.
 6. The artificial disc of claim 1, wherein the superior endplate comprises a socket for removably attaching to a holder tool, and the inferior endplate comprises a socket for removably attaching to a holder tool.
 7. The artificial disc of claim 1, wherein: the superior endplate comprises at least one of titanium, Cr—Co—Mo alloy, or PEEK; the superior core comprises at least one of titanium, Cr—Co—Mo alloy, or PEEK; the inferior core comprises polycarbonate urethane; and the inferior endplate comprises at least one of titanium, Cr—Co—Mo alloy, or PEEK.
 8. An artificial disc comprising: a superior endplate comprising a bi-convex superior surface and a concave inferior surface; a superior core comprising a convex superior surface configured to contact the concave inferior surface of the superior endplate; a flexible inferior core having an upper wall and a lower wall, the flexible inferior core configured to connect to the superior core, the upper wall and the lower wall divided by a mating groove, wherein the flexible inferior core comprises a mating knob surrounded by a groove and a raised ridge on the upper wall extending around an outer periphery of the inferior core for mating with the superior core, the groove being positioned between the mating knob and the raised ridge, wherein the flexible inferior core further comprises a substantially concave superior surface within which the mating knob resides; and an inferior endplate configured to connect to the inferior core, the inferior endplate comprising a bi-convex inferior surface, wherein the superior core includes an inferior mating surface with a mating socket, wherein the mating socket is narrower at its inferior end and wider at its superior end, the mating socket configured to receive the mating knob.
 9. An artificial disc comprising: a superior endplate comprising a bi-convex superior surface and a concave inferior surface; a superior core comprising a convex superior surface configured to contact the concave inferior surface of the superior endplate; a flexible inferior core having an upper wall and a lower wall, the flexible inferior core configured to connect to the superior core, the upper wall and the lower wall divided by a mating groove, an inferior endplate configured to connect to the inferior core, the inferior endplate comprising a bi-convex inferior surface, wherein the flexible inferior core comprises a mating knob surrounded by a groove and a raised ridge on the upper wall extending around an outer periphery of the inferior core for mating with the superior core, the groove being positioned between the mating knob and the raised ridge.
 10. The artificial disc of claim 9, wherein the mating knob is wider at a superior end and narrower at an inferior end.
 11. The artificial disc of claim 9, wherein the inferior core is configured to be compressed slightly when the artificial disc is inserted in an intervertebral space.
 12. The artificial disc of claim 9, wherein the upper wall of the inferior core extends above a superior surface of the inferior endplate.
 13. The artificial disc of claim 9, wherein an outer diameter of the superior core is smaller than an outer diameter of the inferior core.
 14. The artificial disc of claim 9, wherein the mating groove is centrally positioned between the upper wall and the lower wall of the inferior core. 